1. Field of the Invention
This invention is directed toward the measure of properties of earth formation, and more particularly directed toward nuclear measuring systems and the correction of the formation property measurements for adverse effects of instrument standoff from the borehole wall. The invention can be used to make formation property measurements while drilling the borehole, or subsequent to drilling using wireline techniques.
2. Background of the Art
Essentially all nuclear instrument systems used to measure earth formation parameters from within a well borehole are adversely affected by borehole conditions. Borehole conditions include borehole fluid type, borehole irregularity, and the size of the spacing or xe2x80x9cstandoffxe2x80x9d between the downhole measuring instrument and the wall of the borehole. To increase the accuracy, it is necessary to correct measured parameters for borehole conditions including standoff. This correction process is commonly known as xe2x80x9cstandoff correctionxe2x80x9d.
Nuclear measurement systems have been used for decades to measure various properties of earth formation penetrated by a well borehole. The first systems used downhole instruments or xe2x80x9ctoolsxe2x80x9d which were conveyed along the borehole by means of a xe2x80x9cwirelinexe2x80x9d cable. In addition, the wireline served as a means of communication between the downhole tool and equipment at the surface, which typically processed measured data to obtain formation parameters of interest as a function of depth within the borehole. These measurements, commonly referred to as xe2x80x9cwell logsxe2x80x9d or simply xe2x80x9clogsxe2x80x9d, include measures of formation natural gamma radiation, thermal neutron flux, epithermal neutron flux elastic and inelastically scattered neutron, capture gamma radiation, scattered gamma radiation, and the like. A variety of formation parameters are obtained from these measurements, or combinations of these measurements, such as shale content, porosity, density, lithology and hydrocarbon saturation. Most of these nuclear wireline measurements are adversely affected by the borehole. Standoff of the instrument from the borehole wall is an almost universal problem considering the inherently shallow radial depth of investigation of nuclear logging systems.
Wireline systems use a variety of mechanical means to minimize standoff by forcing the tool against the borehole wall. As examples, a prior art neutron porosity tool typically use a bow spring to forced the tool against the borehole wall thereby minimizing standoff effects. A typical scattered gamma ray density tool is constructed with a gamma ray source and one or more gamma ray detectors in a xe2x80x9cpadxe2x80x9d which is mechanically forced against the borehole wall to again minimize standoff effects. Even though controlling the physical position of a wireline nuclear tool within the borehole aids in minimizing borehole effects including standoff, other tool design and data processing techniques are used to further reduce these adverse effects. As examples, neutron porosity and density tools typically use two or even more radiation detectors at different spacings from a neutron or density source, respectively. Detector responses are then combined using a variety of algorithms to further minimize borehole effects in the final computed parameter of interest. As an example, a dual detector processing method known as the xe2x80x9cspine and ribxe2x80x9d technique was introduced in the 1960""s as a means for compensating dual detector density wireline logs for the effects of standoff. This technique relies solely on the response of the two downhole detectors and a tool calibration to compensate for small tool standoffs which are not overcome by mechanical means. The method is effective for standoff magnitudes of generally less than one inch. For larger standoffs, the spine and rib system, used alone, is not an effective compensation means.
Wireline logging is applicable only after the borehole has been drilled. It was recognized in the 1960s that certain operational and economic advantages could be realized if drilling, borehole directional, and formation properties measurements could be made while the borehole is being drilled. This process is generally referred to as measurement-while-drilling (MWD) for real time drilling parameters such as weight on the drill bit, borehole direction, and the like. Formation property measurements made while drilling, such as formation density, are usually referred to as logging-while-drilling (LWD) measurements. The LWD measurements should conceptually be more accurate than their wireline counterparts. This is because the formation is less perturbed in the immediate vicinity of the borehole by the invasion of drilling fluids into the formation. This invasion alters the virgin state of the formation. This effect is particular detrimental to the more shallow depth of investigation nuclear logging measurements.
For brevity, only LWD systems will be discussed. The tools are typically mounted within a drill collar near a drill bit which terminates the lower end of a drill string. The diameter of the drill collar in typically smaller than the diameter of the drill bit. This factor, along with the fact that there is usually a certain amount of drill string xe2x80x9cwobblexe2x80x9d and xe2x80x9cbouncexe2x80x9d during drilling, results in a borehole with diameter greater than the bit gauge, which in turn results in varying standoff between the LWD instrument and the borehole wall. Furthermore, circulation of drilling fluid in the borehole tends to wash out or enlarge the borehole causing still greater and more unpredictable standoff. Even though the major elements of most LWD tools are mounted near the periphery of the drill collar and typically within one or more collar stabilizer fins, standoffs can be quite large and can change dramatically with each rotation of the drill string. Mechanical means such as bow springs and powered pad mandrels used in wireline counterparts are obviously not applicable as a means for minimizing standoff in a rotating drill string. Again for brevity, a prior art xe2x80x9cgammaxe2x80x94gammaxe2x80x9d density LWD system using preferably two gamma ray detectors will be discussed. Operational concepts of this type of density tool are known in the art. It should be understood, however, that many of the previously discussed difficulties and limitations are applicable to other prior art nuclear logging systems such as neutron porosity. LWD formation density systems differ from their counterparts in wireline by the fact that the measurement is made while the tool is rotating with the drill string, thereby causing varying standoff between the tool body and the formation. Typically, an LWD density tool may encounter standoffs anywhere from zero to one inch or greater, depending on the borehole shape and tool configuration. Count data from the preferably two axially spaced detectors are usually sampled at a much faster rate than rotational time, which results in fairly constant standoff per sample. Short sample time periods are used to increase the accuracy of the measurement. However, due to the short sampling time and the statistical nature of the measurement, the detector counts in each sample do not have sufficient statistical precision to apply a statistically meaningful standoff correction such as a spine and rib correction. To improve statistical precision of the measurement and subsequent corrections, count data are summed over a radial segment of the borehole before a standoff correction is applied. Although this improves statistical precision, accuracy is sacrificed.
In attempt to retain both statistical precision and accuracy, prior art LWD density, neutron porosity, and other types of nuclear tools often use independent systems to measure the radial shape of the borehole. This is often referred to as a xe2x80x9ccaliperxe2x80x9d of the borehole. Caliper measurements are then combined with detector count data and a spine and rib or other type of correction algorithm to obtain borehole compensated density or other parameter of interest. The caliper is, in itself, a complete and self contained LWD tool with all of the associated complexity, initial cost, operational cost, and reliability problems. The caliper system, unlike wireline counterparts, can not make physical contact with the borehole wall since the drill string is typically rotating. Ultrasonic transducers are commonly used in LWD caliper systems. They define the shape of the borehole by measuring travel time of acoustic waves emitted from the transducer, reflected by the borehole wall, and subsequently detected by the transducer.
U.S. Pat. Nos. 5,397,893 and 5,091,644 to Daniel C. Minnette are related and disclose an LWD tool which measures formation density. An error minimization method is used to sort or xe2x80x9cbinxe2x80x9d count data from two detectors before applying a standoff correction. In a first embodiment, the borehole cross section is radially segmented and preferably divided into four quadrants. Counts recorded in each quadrant are sorted over multiple rotations of the tool by quadrant in which they are measured. The sums of sorted counts are subsequently combined to obtain a density measurement for a particular borehole segment, which is compensated to some degree, for standoff within that segment. Segmenting of the borehole is predetermined in that the system is initially set up to record counts preferably within 90 degree segments as the tool rotates within the borehole. Alternately, the borehole can be partitioned in a different manner if borehole conditions are thought to warrant such partitioning. As an example, boreholes known to be very radially irregular might warrant smaller segments, such as 45 degrees. The decision must be made before the LWD operation is initiated. A second embodiment of the Minnette system teaches a measurement from an independent borehole caliper, such as an acoustic caliper, to define the count sorting scheme prior to correcting density measurements for standoff. This overcomes the problem of changing borehole conditions, but adds complexity and cost to the system in the form of a stand-alone LWD borehole calipering system.
U.S. Pat. No. 5,486,695 to Ward E. Schultz discloses an LWD nuclear logging system which used an independent sensor, such as an acoustic transducer, to measure tool standoff. The system uses a method similar to Minnette""s to correct for standoff by sorting or xe2x80x9cbinningxe2x80x9d detector count data as a continuous rather than a discrete function of standoff. The independent tool standoff measurement is then combined with detector count data using various weighting schemes to obtain formation parameters of interest which have been compensated for standoff. The system requires an independent tool standoff measuring system which, again, adds cost and complexity to the LWD.
U.S. Pat. No. 5,473,158 to Jaques M. Holeuka et al discloses methods and apparatus for measuring density, neutron porosity, and other formation parameters using a LWD system. One embodiment of the system uses ultrasonic transducers to determine standoff, and the standoff measurement is used to correct porosity, density and other parametric measurements for adverse effects of tool standoff. Once again, an independent borehole caliper instrument is required for the standoff measurement. A quadrature method of azimuthally partitioning the borehole is used in an alternate embodiment to detector responses from predetermined angular segments of the borehole. As an example, the borehole is divided into 90 degree segments, and means such as directional equipment (accelerometers, magnetometers and the like) or even count rate processing are used to define detector signals from the xe2x80x9cbottomxe2x80x9d, the xe2x80x9ctopxe2x80x9d, and the xe2x80x9csidesxe2x80x9d of the borehole. The measurement from the segment with the least standoff is then chosen to be the best measurement. This introduces additional statistical uncertainty into the measurement in that significant amounts of measured data are not used to compute the parameters of interest.
U.S. Pat. No. 5,451,779 to Ronald L. Spross et al discloses a LWD density system which uses a xe2x80x9cstatistical flagxe2x80x9d to determine when density measurements should be corrected for standoff. The flag compares actual statistical variations of measured count rates with theoretical statistical variations for the tool operating in a standard or gauge bore hole. The system requires control and monitoring of the rate of rotation of the tool in order to make meaningful comparisons between measured and theoretical statistical variations. If the difference between the two variations exceeds a predetermined value, an irregular section of borehole is xe2x80x9cflaggedxe2x80x9d and appropriate corrections are made to the measured data using the spine and rib technique or other similar data correction schemes. In an alternate embodiment, an acoustic borehole caliper is used to flag irregular borehole conditions or tool standoff thereby requiring dedicated borehole calipering system with accompanying increase in hardware complexity and cost. Once a section of irregular borehole or a standoff condition is flagged, other means such as the spine and rib technique are used to correct the measured count rates, and to thereby obtain a borehole compensated density measurement.
U.S. Pat. No. 5,250,806 to Erik Rhein-Knudsen et al discloses apparatus for measuring the formation parameters density, neutron porosity, and photoelectric absorption (PF) factor. The system is embodied as a LWD tool, and no data processing methods are discussed. The density portion of the tool comprises a gamma ray source mounted eccentrically within a drill collar housing and two longitudinally spaced gamma ray detectors located in blade comprising a first set of stabilizer blades. The neutron porosity portion of the tool comprises a neutron source mounted concentrically within the drill collar housing and preferably two longitudinally spaced secondary radiation detectors (either neutron of gamma ray) located in a blade comprising a second set of stabilizer blades. PF is obtained from an energy spectrum of one or both of the gamma ray detectors in the density section. The preferred embodiment of the invention includes a pair of blade mounted, opposing ultrasonic transducers used to determine standoff which, in turn, is used to correct the neutron porosity and density measurements for adverse effects of tool standoff. There is no specific teaching of standoff correction methods with or without ultrasonic borehole size parametric measurements. The patent is directly only toward apparatus.
U.S. Pat. No. 5,175,429 to Hugh E. Hall, Jr. discloses a secondary LWD measurement system for determining tool displacement from the borehole wall, and the subsequent use of this displacement or standoff measurement to correct nuclear parametric measurements, such as density and neutron porosity, for adverse effects of standoff.
This invention is directed toward nuclear measurements within a well borehole. The invention is applicable to logging-while-drilling (LWD) and to wireline systems. The disclosure is applicable to both LWD and wireline systems which provides a measure of one or more parameters of interest of an earth formation penetrated by a borehole. The measurements are corrected for adverse effects of the borehole including tool standoff from the borehole wall.
The invention is applicable to a variety of nuclear logging systems. Emphasis will, however, be directed toward the system embodied as a combination LWD density and neutron porosity tools. The neutron porosity tool can be used to determine other parameters, such as formation. For this reason, the neutron porosity tool will be referred to as the xe2x80x9cneutronxe2x80x9d tool, with the understanding that formation porosity is only one of several parameters of interest that can be measured using the tool. The density tool is preferably in close proximity to the neutron tool. The response of the density tool can be used to correct the response of the neutron porosity tool for adverse borehole effects including borehole size, drilling fluid weight, drilling fluid salinity and tool standoff.
Attention is first directed toward the density measurement. For purposes of discussion, the density tool will be embodied as a dual detector formation density tool comprising a preferably isotopic source of gamma radiation and two longitudinally spaced gamma ray detectors. The source and detectors are preferably mounted within the wall of a drill collar, hereafter referred to as the xe2x80x9ctoolxe2x80x9d, which is mounted in a drill string in the vicinity of a drill bit. The tool rotates as the drill string is rotated thereby allowing the drill bit to advance the borehole. Tool rotation is not a necessary requirement to operate the invention. The invention is also operable when a downhole motor is used to rotate the drill bit, and the tool is above this rotating portion of the drill string.
As a brief summary of operating concepts of a dual detector density tool is presented so that the present invention can be more easily understood. Gamma radiation is emitted by the source, passes through any material between the tool and the borehole wall, enters the formation where it interacts with material within the formation, and a portion of the radiation is scattered back into the borehole at a reduced energy. A portion of radiation scattered back into the borehole is recorded by the detectors. Source gamma ray energy is selected so that the primary mode of reaction is Compton scatter, which is related to the electron density of the composite formation material including the formation matrix material and any fluid filling pore space within the matrix. Electron density is, in turn, is related to the xe2x80x9cbulkxe2x80x9d density of the formation. The count rate measured by the tool detectors can, therefore, be related to the formation property of interest, which is bulk density. This relationship is determined by calibrating the tool under known borehole and formation conditions, and is known in the art. Gamma radiation not only interacts with the formation, but also with any intervening material between the tool and the borehole wall. This intervening material includes borehole fluid and particulate material, known as xe2x80x9cmudcakexe2x80x9d, which builds up on the borehole wall due to invasion into the formation of borehole fluid. Mudcake and any other intervening material adversely affect the bulk density measurement. Two detectors are used to minimize the effects of mudcake and tool standoff. The responses of the two detectors can be combined to minimize the effects of standoff and mudcake using a variety of algorithms including the spine and rib algorithm. The effects of standoff are typically much more severe in nuclear LWD systems than in wireline systems, even though mudcake buildup is minimal and the borehole fluid type is constant or at least slowly varying with the drilling operation. An LWD density tool typically rotates continuously inside the borehole as the measurement is being made. If the tool rotates centralized within a borehole which is necessarily larger than the tool, detector counts rates will be constant as a function of time for a given formation density and borehole diameter. If the standoff is perhaps one inch or less, and the counts collected are statistically significant, the spine and rib method can be used to compensate for standoff, and an accurate measure of formation density can be obtained. Rarely in practice does the tool rotate concentrically within the borehole. Since most boreholes are not exactly vertical, and considering the torque on the drill bit and the flexibility of the drill string, the tool is most likely rotating against one side of the borehole. Recall that the source and detectors are preferably located within the wall of the tool. During a complete rotation of the tool, standoff that the density tool xe2x80x9cseesxe2x80x9d varies from essentially zero when the tool is oriented so that the source and detectors are facing a tool contact point on the borehole wall, increases to a maximum 180 degrees later when the source and detectors are facing the direction, and then decreases to essentially zero when the tool is again oriented so that the source and detectors are again face the borehole wall. Measured count rate is now a function of formation density, borehole diameter, and a variable standoff caused by the eccentric rotation of the tool within the borehole. A simple spine and rib analysis of the count rate data will yield statistically insignificant measurements or inaccurate measurements, or both, as will be detailed in a following section of this specification.
The present LWD density system collects or xe2x80x9csamplesxe2x80x9d counts in each detector as the tool rotates. The sample time interval is selected to be relatively short when compared with the time required for one complete rotation of the tool. By selecting a short sample time interval, tool standoff varies insignificantly during the sampling. The short sample time interval results, however, in a count sample measure with a large statistical error. This would yield a statistically insignificant bulk density measurement if the spine and rib technique were applied to each set of detector count sample measurements. To overcome the statistics problem, samples are collected and stored for a sample period that typically includes several hundred or more count sample intervals and several rotations of the tool. It is desirable to make the sample period sufficiently long to minimize statistical error in the final density measurement, but also sufficiently short so that the change in true formation density seen by the tool is minimal. Samples from each detector are sorted by magnitude, and a running sum or integral of count rate data is performed over the sample period. If the tool is operating centered within the borehole, a plot of integrated counts as function of time will be a straight line over a selected sample period, with the slope of the line representing detector count rate which, in turn, represents an xe2x80x9capparentxe2x80x9d density. Apparent densities are processed preferably using the spine and rib technique to obtain bulk density compensated for standoff. If the tool is operating eccentered within the borehole, integrated counts sorted by magnitude and plotted as a function of time over the sample period will be a non-linear curve. In this case, which as mentioned above, is typical a plurality of contiguous, straight line segments is fitted to the nonlinear curve using predetermined fitting criteria. Detector count sample measurements within each segment are summed, and combined with a time duration of the segment to obtain a segment count rate. Apparent density for the segment is then determined for a standoff over an azimuthal segment of the borehole. The use of count rate and subsequently apparent density from a fitted straight line segment is far more statistically significant than using count rate and associated apparent density from a single detector sample within the segment. Apparent densities obtained from the fitted straight line segments are then processed using the spine and rib or other correction technique to obtain a borehole compensated density for each segment. Weighting is used to emphasize measurements with minimal statistical uncertainty, and compensated densities from all segments are then combined to yield a composite, compensated bulk density which is accurate and statistically significant. Alternate means for determining density and standoff from the density tool response will be discussed in subsequent sections of this disclosure.
Attention is now briefly directed toward the neutron tool. The neutron tool responds mainly to the hydrogen content of the borehole environs. If it is assumed that most hydrogen is contained in pore space of earth formation and not in the rock matrix, then a tool response to hydrogen concentration can be related to formation porosity. Unfortunately, hydrogen is usually also present in fluid within the borehole, therefore the neutron tool response is also affected by borehole size, drilling mud weight, drilling mud salinity, and tool standoff. Porosity measurements must be corrected for these effects in order to obtain accurate parametric measurements. Previously discussed standoff data obtained from the density measurement can be used to correct the neutron tool response for borehole size and standoff effects.
It is known in the art that neutron tools respond to other elements and conditions, such as boron typically found in shales. The basis concept of the neutron prosity measurement is, however, based upon the neutron tool""s response to hydrogen.
It is noted that the invention does not require a rotating tool. In cases where a downhole motor such as xe2x80x9cmudxe2x80x9d motor is used to rotate the bit, or the density measurements are made as the drill string is being withdrawn from the borehole, techniques disclosed in this specification are equally applicable. A rotating drill string and tool are used for purposed of discussion.
The present LWD system requires no independent borehole calipering means. No predetermined azimuthal segmenting of the well borehole is required, but the system can be used to measure azimuthal density values in predetermined segments of the borehole if desired. The system is automatic and operates effectively when the tool is operating centered or eccentered within the borehole. Although the system has been summarized configured as a compensated density tool, the basic concepts are applicable to other nuclear LWD logging systems such as formation photoelectric factor measurements.